Diesel engine exhaust contains particulate matter, the emission of which is regulated for environmental and health reasons. This particulate matter generally constitutes a soluble organic fraction (“SOF”) and a remaining portion of hard carbon. The soluble organic fraction may be partially or wholly removed through oxidation in an oxidation catalyst device such as a catalytic converter; however, this typically results in a reduction of only about 20 percent of total particulate emissions. Thus, vehicles equipped with diesel engines may include diesel particulate filters for more completely removing the particulate matter from the exhaust stream, including the hard carbon portion. Conventional wall flow type diesel particulate filters may have particulate removal efficiencies of about 85 percent. However, diesel particulate filters, particularly those that have relatively high particulate filtration efficiency, are generally associated with high back pressures because of the restriction to flow through the filter. Further, with use, soot or other carbon-based particulate matter accumulates on the diesel particulate filters causing the buildup of additional undesirable back pressure in the exhaust systems. Engines that have large particulate mass emission rates may develop excessive back pressure levels in a relatively short period of time. High back pressures decrease engine efficiency and reduce engine performance. Therefore, it is desired to have diesel particulate filtration systems that minimize back pressure while capturing a high percentage of the particulate matter in the exhaust.
Conventional wall flow diesel particulate filters (DPFs) are high particulate removal efficiency filters that include a porous-walled honeycomb substrate (i.e., monolith) with channels that extend generally from an upstream end to a downstream end of the substrate. Generally half the channels are plugged adjacent the downstream end of the substrate and the other half of the channels are plugged adjacent the upstream end of the substrate. This plugged configuration forces exhaust flow to pass radially through the porous walls defining the channels of the substrate in order to exit the diesel particulate filter.
To prevent diesel particulate filters from becoming excessively loaded with particulate matter, it is necessary to regenerate the diesel particulate filters by burning off (i.e., oxidizing) the particulates that accumulate on the filters. It is known to those of skill in the art that one method by which particulate matter may be oxidized is to raise the temperature of the exhaust gas sufficiently to allow the excess oxygen in the exhaust gas to oxidize the particulate matter. Also well-known to those of skill in the art is that particulate matter may be oxidized at a lower temperature in the presence of sufficient amounts of nitrogen dioxide (NO2).
Diesel exhaust inherently contains nitrogen oxides (NOx), which consist primarily of nitric oxide (NO) and nitrogen dioxide (NO2). Typically, the NO2 inherently present in the exhaust stream is a relatively small percentage of total NOx, such as in the range of 5 to 20 percent but usually in the range of 5 to 10 percent. Although some regeneration of a diesel particulate filter occurs at such levels, it is insufficient to result in complete regeneration. The effectiveness of NO2 in regenerating a particulate filter depends in part on the ratio of NOx to particulate matter in the exhaust stream. Generally, the reaction of “2NO2+C═CO2+2NO” requires 8 times NO2 per unit of C in mass.
To promote full regeneration, it is often necessary to increase the quantity of NO2 in the exhaust stream. This is particularly true where the NOx/particulate ratio is relatively small. One method to produce sufficient quantities of NO2 is to use an oxidation catalyst to oxidize a portion of the NO present in the exhaust stream to NO2. For example, a catalytic converter including a diesel oxidation catalyst can be positioned upstream from the diesel particulate filter and/or the diesel particulate filter itself can include a diesel oxidation catalyst. However, these types of prior art arrangements may result in excessive NO2 emissions.